The items are all fulfilled by Amazon so they know at the time of order what to promise, and also that it is Prime "2 day" shipping. As mentioned sometimes items come really quick, but it seems the ones you want the most are the ones delayed the most. (Of course those are also what you are paying most attention to.)

My memory of prior years is that the vast majority of items arrived within time, and that it has been getting worse over the last year or so.

Another interesting case is when you select the slowest shipping option and it shows up in 24-48 hours anyway. Happens all the time.

Same here, both now (in Chicago) and previously (in Boston). I think there are Amazon warehouses nearby...